The only way to tell with reasonable certainty of what you're getting is to have the product tested, ideally at multiple independent, high quality laboratories. Even then, if you're testing blotter or microdots, you're only going to know about the ones you tested, not about the rest of the batch. Properly shaken/stirred liquid should be better in that if one drop tested fine, the rest should be the same. But even then, if the substance easily separates out in the solution, you can't trust that either.
If anyone with a deeper knowledge of chemistry could speak about the effectiveness of testing, that would be welcome.